Defective transformation of chromosomal markers in DNA polymerase I mutants of the radioresistant bacterium Deinococcus radiodurans.

The transformation efficiency of six independently selected chromosomal markers (four for rifampicin resistance and two for acriflavine resistance) was found to be reduced by about 3 logs in a Deinococcus radiodurans strain that was isogenic with wild type except for an insertional mutation in the pol gene that eliminated DNA polymerase I activity (strain 6R1A). D. radiodurans strains UV17 and 303, previously obtained by chemical mutagenesis, were determined to be partially deficient in DNA Pol I activity as assessed in a permeabilized cell system. Both UV17 and 303 demonstrated intermediate transforming efficiencies that correlated with their levels of residual polymerase activity. The transformation efficiency of strain 6R1A could be greatly restored by expression of cloned E. coli DNA Pol I, but not to wild-type levels. Plasmid transfer and chromosomal duplication insertion were not substantially affected by lack of DNA Pol I activity. D. radiodurans is known to possess extraordinarily efficient repair pathways for DNA damage, and is refractory to DNA damage-induced mutagenesis caused by numerous agents, including several that cause base mispairing. We suggest that D. radiodurans may differ from other naturally transformable bacteria in that DNA Pol I is needed to efficiently convert most drug-resistance markers. This unusual mechanism may be required to accomplish chromosomal conversion prior to correction of donor DNA by this organism's efficient repair pathways.